This invention relates generally to refrigeration and, more particularly, to refrigeration for high temperature superconductivity applications.
Superconductivity is the phenomenon wherein certain metals, alloys and compounds lose electrical resistance so that they have infinite electrical conductivity. Until recently, superconductivity was observed only at extremely low temperatures just slightly above absolute zero. Maintaining superconductors at such low temperatures is very expensive, typically requiring the use of liquid helium, thus limiting the commercial applications for this technology.
Recently a number of materials have been discovered which exhibit superconductivity at higher temperatures, such as in the range from 15 to 75 K. While such materials may be kept at their superconducting temperatures using liquid helium or very cold helium vapor, such a refrigeration scheme is quite costly. Unfortunately liquid nitrogen, a relatively low cost way to provide cryogenic refrigeration, cannot effectively provide refrigeration to get down to the superconducting temperatures of most high temperature superconductors.
An electric transmission cable made of high temperature superconducting materials offers significant benefits for the transmission of large amounts of electricity with very little loss. High temperature superconducting material performance generally improves roughly an order of magnitude at temperatures of about 30 to 50 K from that at temperatures around 80 K which is achieved using liquid nitrogen.
The application of superconducting systems such as cable, transformer, fault current controller/limitor and others is dependent in part on the development of economic refrigeration systems. Superconducting systems need to be maintained at temperatures in the range of 4 to 80 K. However, the system needs to be shielded from heat leak starting at ambient temperature down to the operating temperature of the superconducting system. Refrigeration below liquid nitrogen temperatures becomes excessively expensive, as the temperature gets lower when compared to liquid nitrogen level refrigeration. Liquid nitrogen level refrigeration is considerably less expensive but is not cold enough for-most-high temperature superconductivity applications.
Accordingly, it is an object of this invention to provide a method for refrigerating a high temperature superconducting device which requires less power and thus less cost than heretofore available systems.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for cooling a high temperature superconducting device comprising:
(A) providing a high temperature superconducting device operating at a temperature within a high temperature superconductivity temperature range of from 20 to 80 K;
(B) cooling a first heat transfer means to a first temperature which exceeds the temperature of saturated liquid nitrogen, and warming the cooled first heat transfer means by intercepting ambient heat from passing to the high temperature superconducting device; and
(C) cooling a second heat transfer means to a second temperature within the high temperature superconductivity temperature range, and warming the cooled second heat transfer means by heat exchange with the high temperature superconducting device to maintain the high temperature superconducting-device within the high temperature superconductivity temperature range.
As used herein, the term xe2x80x9chigh temperature superconducting devicexe2x80x9d means an electrical device such as a cable, transformer, fault current controller/limitor or magnet, in which the electrical resistance to the passage of current is reduced to essentially zero while being maintained at superconducting temperatures.